1. Field of the Invention
The present invention relates to a radio receiving apparatus having a plurality of antenna elements, and a radio receiving method, and more particularly to a radio receiving apparatus in which a single control signal controls the gains of signals received by a plurality of antenna elements, and a radio receiving method employed in the apparatus.
2. Description of the Related Art
In general, receiving apparatuses for performing demodulation using digital signals must have an A/D converter. Most A/D converters convert, into digital signals, analog signals of only a certain power range. If an analog signal cannot appropriately be converted into a digital signal since its power exceeds the range, the resultant digital signal is distorted. It is well known that this distortion significantly degrades the receiving characteristic. To avoid distortion and maintain a good receiving characteristic, it is necessary to appropriately adjust the gain of an analog signal to be input to an A/D converter.
When there are a plurality of antenna elements, a plurality of gain adjusting devices are needed for adjusting the respective gains of signals received by the antenna elements. If the gain adjusting devices can be controlled individually, it is sufficient if each device adjusts the gain of a signal to be input to a corresponding A/D converter. However, if a plurality of gain adjusting devices must be controlled by a single control signal, it is difficult to appropriately control all input signals since the input signals may be adjusted using different gain-adjusting values.
As a technique for adjusting a plurality of gain adjusting devices using a single control signal, Jpn. Pat. Appln. KOKAI Publication No. 2003-152611, for example, has proposed a receiving apparatus in which a plurality of antenna elements and gain adjusting devices are employed, and all the gain adjusting devices are controlled by a single control signal.
However, if the amplitudes of analog signals received by the antenna elements are reduced, as in the publication, to values that enable the A/D converters to appropriately convert the analog signals into digital signals, the receiving characteristic of a synthetic signal obtained by synthesizing a plurality of digital signals corresponding to the analog signals may degrade.
An example case will be described. General radio sets have a noise floor that does not depend upon the reception power, therefore even if the reception power is increased to some extent, an increase in signal-to-noise ratio (SNR) is suppressed. Assume that one of the antenna elements receives a signal of a very high power, and the remaining antenna elements (two or more antenna elements) receive signals of power that falls within a range in which increases in SNR is suppressed. The receiving characteristic of the finally processed signal may be higher in the case where gain control is performed based on the antenna elements having substantially the same SNR as the maximum power antenna, and the signals received by these antenna elements are synthesized, than in the case where gain control is performed based on the maximum power antenna element, and therefore the remaining antenna elements cannot be used.